1. Field
The invention relates to antenna systems and in particular to antenna systems used in radar applications.
2. Description of the Related Art
The resolution of all lens-based instruments is defined by the finite dimensions of the electromagnetic wave used by a lens. The resolving power, or the minimum separation between two points which can be resolved (dmin), can be approximated by λ/2, where λ is the wavelength of light. This limitation is the result of diffraction that takes place because of the wave nature of electromagnetic radiation. Moreover, it should be noted that the resolution limit described above arises from the assumption that the image of an object is being detected in the “far-field”, that is, a distance at which the far-field dominates over all other aspects of the electromagnetic radiation. For these reasons, where resolution is limited to no better than one-half of the wavelength of electromagnetic radiation being used, such imaging is termed “far-field” imaging or diffraction-limited viewing.
Conventional far-field imaging techniques are unable to use the “near-field” component of the radiated field. The near-field component of a radiated field is a standing wave, as compared to the traveling wave of the far-field component. As such, the near-field is evanescent in conventional systems because it decays very rapidly as distance increases from the object of interest, often at r3 or greater, where r is the distance from the object of interest. For several types of radio antennas, it should be noted that although the near-field exists, its amplitude is significantly weaker than the far-field for R>[2D2]/λ, where λ=the wavelength, D=the antenna size, and R=the radial distance from the antenna. The boundary between the near-field and the far-field is conservatively estimated at approximately λ/16 from the antenna or object of interest.
An increasingly important and rapidly developing alternative to conventional lens-based imaging is the “near-field” technique which provides superresolution imaging. The term “superresolution” defines any means for optical imaging or spectroscopy that permits spatial resolution which exceeds the diffraction limitation caused by the wave nature of electromagnetic energy; and provides a resolution which is less than one-half the wavelength of the light actually being used. All superresolution near-field imaging and scanning near-field optical microscopy (“SNOM”) is based on the fact that although light cannot be focused to a spot less than one-half the wavelength of light (λ/2), it can be directed through a device or article which reduces the size of the light energy to dimensions smaller than λ/2 via near-field detection.
The basic principle of near-field viewing and imaging is best illustrated by the aperture technique as is illustrated by FIG. 1. When light is directed through a subwavelength (i.e. sub-λ) sized hole, the portion of energy that passes through the hole will at first be confined to the dimensions of the aperture. The exiting light being of subwavelength dimensions will then diffract; however, there will be a distinct region in the vicinity of the aperture called the “near-field” where the existing light beam retains the approximate dimensions of the hole. If this subwavelength light beam within the near-field region is used to raster scan the surface of an object, a two-dimensional image can be created in a serial fashion (one point at a time).
In addition to near-field imaging techniques, image resolution can be improved by using metamaterials. Metamaterials are artificial media with unusual electromagnetic properties that result in negative permittivity ∈, permeability μ, and/or negative index of refraction N (N=√(∈μ)), that are controlled by the design of the material. Metamaterials are well-known by those of skill in the art and their theory and construction is beyond the scope of what will be discussed here. For more information on the theory of metamaterials and their uses, the reader is directed to the following sources: T. W. Ebbesen, H. J. Lezec, H. F. Ghaemi, T. Thio, P. A. Wolff, “Extraordinary Optical Transmission through Sub-Wavelength Hole Arrays”, Nature, Vol. 391, pp. 667-69, 1998; Anthony Holden, “Inside the Wavelength: Electromagnetics in the Near Field”, Foresight Directorate; J. B. Pendry, A. J. Holden, D. J. Robbins, and W. J. Stewart, “Magnetism from Conductors and Enhanced Nonlinear Phenomena”, IEEE Trans. Microwave Theory Tech., Vol. 47, No. 11, pp. 2075-2084, November 1999; David Smith, “Superlens breaks optical barrier”, PhysicsWeb, August 2005; N. Fang, X. Zhang, “Imaging Properties of a Metamaterial Superlens”, Appl. Phys. Lett., Vol. 82, No. 2, pp. 161-163, January 2003; 23. Nassenstein, H. Phys. Lett. 29a, 175 (1969); Chew, H., D.-S. Wang and Kerker M. Appl. Opt. 18, 2679 (1979); Wolf, E. and Nieto-Vesperinas, M. J. Opt. Soc. Am. A 2, 886 (1985).
It may be helpful to provide a tangible example of the effects of metamaterials by comparing how a beam of light is affected by a material with a negative refractive index (a metamaterial) and a material with a positive refractive index, such as glass. If the light beam strikes the surface of the material at an acute angle relative to the surface, the beam of light will enter the material and refract, or bend, away from the angle at which it entered the material. In general, a conventional material having a positive refractive index, such as glass, the beam of light will refract slightly toward the normal (i.e. an imaginary line perpendicular to the surface of the material at the point of the beam's entry), and continue through the material on the opposite side of the normal. However, with a metamaterial having a negative refractive index, the beam of light will refract greatly by staying on the same side of the normal. It will be appreciated that this has profound applications for near-field imaging. Because the near-field decays so rapidly, if it is combined with a metamaterial having a negative refractive index, the rapid decay can be controlled by placing a metamaterial within the near-field, thereby allowing the near-field component to be captured. Once the near-field is captured, it is possible to process both the far-field and the near-field components to dramatically improve the image resolution.
Information relevant to attempts to use metamaterials in antenna systems can be found in U.S. Pat. Nos. 6,958,729 and 7,218,285. However, both of these references suffers from one or more of the following disadvantages: not enhancing the near-field component of the wave, not using near-field probes, not using sub-wavelength illumination, not utilizing for metal penetration, and not detecting the near-field of the source or object to be imaged.
For the foregoing reasons, there is a need for an improved resolution radar system that can enhance the near-field component of the wave, use near-field probes, and use sub-wavelength illumination.
The art referred to and/or described above is not intended to constitute an admission that any patent, publication or other information referred to herein is “prior art” with respect to this invention. In addition, this section should not be construed to mean that a search has been made or that no other pertinent information as defined in 37 C.F.R. §1.56(a) exists.
All U.S. patents and applications and all other published documents mentioned anywhere in this application are incorporated herein by reference in their entirety.